1. Field of the Invention
The present invention relates generally to a supported silver catalyst useful for the vapor-phase oxidation of ethylene to ethylene oxide and the method of producing ethylene oxide. More particularly, the invention relates to a supported silver catalyst containing an alkali metal such as cesium. The present invention also relates to a method of preparing an improved supported silver catalyst containing such an alkali metal and exhibiting increased activity and selectivity.
2. Related Art
The use of supported silver catalysts for the oxidation of ethylene to ethylene oxide has been long known in the art. Additionally, over the years various promoting metals have been added to further enhance performance. In particular, the use of alkali metals has been disclosed in various amounts and added by different methods. A very extensive review of the patent literature is given in G.B. No. 2,043,481A. Such disclosures have been somewhat inconsistent in their teachings, as can be seen by comparing U.S. Pat. No. 2,238,474 in which sodium and lithium hydroxides were suggested as promoters and potassium and cesium were shown to be poisons to U.S. Pat. No. 2,671,764 where rubidium and cesium sulfates were suggested as promoting compounds.
Although alkali metals were suggested generally in the earlier disclosures, it is also generally true that more recent workers in the field have considered potassium, rubidium, and cesium as the preferred alkali metals. For example, see the series of patents to Nielson, et al., in which these materials were used in small amounts co-deposited with the silver--U.S. Pat. Nos. 3,962,136; 4,010,115; and 4,012,425. Still more recently the art has emphasized synergistic combinations of the alkali metals. For example see G.B. No. 2,043,241A cited above and U.S. Pat. Nos. 4,212,772 or 4,226,782. The art teaches, in addition that the alkali metals may be used to rejuvenate used catalysts, as for example U.S. Pat. Nos. 4,123,385; 4,033,903; 4,177,169; and 4,186,106. The art teaches that the alkali metals may be deposited either before the silver is placed on the support (pre-deposited)--U.S. Pat. No. 4,207,210; at the same time the silver is deposited (co-deposited)--U.S. Pat. Nos. 4,066,575 and 4,248,740; or subsequent to deposition of silver (post-deposited)--G.B. No. 2,045,636A.
The amount of alkali metal was suggested to be in quite a wide range in the older art. It was often indicated that large quantities, e.g. up to several per cent of an alkali metal could be used. More recently, the art generally has taught that small quantities of alkali metals produce the optimum effect no matter when the silver and the alkali metals were deposited. Kilty in U.S. Pat. No. 4,207,210 related the optimum amount to the surface area of the support. Exceptions to the above include patents issued to ICI which teach the use of large amounts of sodium alone (G.B. No. 1,560,480) and potassium in combination with smaller amounts of cesium and rubidium (U.S. Pat. No. 4,226,782). However, the art generally teaches that the optimum will be found in substantially lower quantities, perhaps on the order of 50-500 ppm by weight. An improved silver catalyst, prepared by impregnating a support with a hydrocarbon solution of a silver salt of a neo-acid having 7 or more carbon atoms is disclosed in commonly assigned U.S. Pat. No. 4,555,501 to Armstrong and subsequently modified by Becker et al in commonly assigned U.S. Pat. No. 4,663,303 both of which are incorporated herein in their entirety.
Commonly assigned U.S. patent application of Rashkin U.S. Pat. No. 4,774,222 teaches that large amounts of alkali metals might be used. However, "reactivation" at higher temperatures for considerable time periods is required. For example, the shortest time period used in the examples is three hours followed by an additional eleven hours although shorter periods are suggested. Likewise, Mitsuhata in U.S. Pat. No. 4,389,338 discloses the use of higher amounts of alkali metals and the "reactivation" at higher temperatures for lengthy periods. For example Mitsuhata requires 12 hours of heat treatment at 600.degree. C.
It has now been found that if the procedure of the present invention is followed, the high temperature treatment period can be reduced significantly, e.g., two hours at 600.degree. C. in lieu of the 12 hours required by Mitsuhata and similar periods used by Rashkin and the catalyst produced may be more stable than the neo-acid catalysts of Armstrong and Becker, et al.